[PATCH v3] drm: do not sleep on vblank while holding a mutex

[Ilija Hadzic]

drm_wait_vblank must be DRM_UNLOCKED because otherwise it
will grab the drm_global_mutex and then go to sleep until the vblank
event it is waiting for. That can wreck havoc in the windowing system
because if one process issues this ioctl, it will block all other
processes for the duration of all vblanks between the current and the
one it is waiting for. In some cases it can block the entire windowing
system.

v2: incorporate comments received from Daniel Vetter and
    Michel Daenzer.

v3: after a lengty discussion with Daniel Vetter, it was concluded
    we should not worry about any locking, within drm_wait_vblank
    function so this patch becomes a rather trivial removal
    of drm_global_mutex from drm_wait_vblank

Signed-off-by: Ilija Hadzic <ihadzic at research.bell-labs.com>
---
 drivers/gpu/drm/drm_drv.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index dbabcb0..dc0eb0b 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -124,7 +124,7 @@ static struct drm_ioctl_desc drm_ioctls[] = {
 	DRM_IOCTL_DEF(DRM_IOCTL_SG_ALLOC, drm_sg_alloc_ioctl, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
 	DRM_IOCTL_DEF(DRM_IOCTL_SG_FREE, drm_sg_free, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
 
-	DRM_IOCTL_DEF(DRM_IOCTL_WAIT_VBLANK, drm_wait_vblank, 0),
+	DRM_IOCTL_DEF(DRM_IOCTL_WAIT_VBLANK, drm_wait_vblank, DRM_UNLOCKED),
 
 	DRM_IOCTL_DEF(DRM_IOCTL_MODESET_CTL, drm_modeset_ctl, 0),
 
-- 
1.7.7